Our broad goals in this research are to study the biological constraints and the role of experience in setting up functionally specialized neural systems in normal development and to study the nature and extent of changes in this process in cases of abnormal development. An important approach in this endeavor is to study changes in brain organization that occur as a function of chronological age and to contrast these with changes linked to specific abilities when age is held constant. The variability occurring in normal development provides one opportunity to address this issue. The study of abnormal development, as in the case of language impaired (LI) children, children with focal brain lesions (FL children) and children with Williams Syndrome, provides another opportunity to link specific changes in neural development with alterations in specific sensory and cognitive abilities. To this end we will record event-related potentials (ERPs) from over several brain regions making comparisons within and between the cerebral hemispheres in a series of studies designed to assess different aspects of sensory, language and cognitive processing. The specific aims of the proposed series of experiments ar to asses the hypotheses that (a) different neural systems mediate semantic and syntactic aspects of language processing from an early age, (b) neural systems important in grammatical processing are more vulnerable to early experience that are the systems that mediate semantic processing and these may be abnormally organized in language impaired children and FL children and Williams Syndrome; (c) abnormal organization of neural systems associated with processing rapidly presented auditory stimuli is linked with abnormal language acquisition in a subset of Li children, and may also be linked to the sparing of language in Williams Ss. (d) to determine whether the functional specializations of the right hemisphere for face recognition are preceded by and depend upon the development of left hemisphere specialization for language and so may be abnormally organized in children with abnormal language acquisition and (e) to examine the timing and organization of neural systems that mediate different aspects of visuo-spatial processing in populations of children who show selective deficits in processing local versus global properties of hierarchical forms.